Printing inks are mixtures of coloring agents dispersed or dissolved in a liquid vehicle or carrier, which forms a paste that can be printed on a substrate and dried. Printing inks may, in general, be divided into four classes: 1) letterpress; 2) lithographic; 3) flexographic; and 4) rotogravure. A varnish is a resinous solution that is spread on wood or metal surfaces to provide a hard, lustrous, generally transparent coating for protection.
Lithographic printing inks are used in a number of printing processes, such as offset lithography, in which the image areas of a plate are treated to accept greasy inks and repel water, while the nonimage areas of the plate accept water and repel ink. A lithographic printing ink is preferably soluble in organic solvents and strongly hydrophobic.
Typically, the coloring agents used in inks are pigments, toners, and dyes, or combinations thereof. Coloring agents provide contrast against a substrate background on which the inks are printed. A liquid resin is frequently used as a vehicle or carrier for the coloring agents during printing operations, and, in most cases, serves to bind the coloring agents to the substrate. One of the most important functions of the liquid resin carrier is to promote pigment dispersion.
The traditional practice in the field of lithographic ink formulation has been to combine a linseed oil with an alkyd resin or phenolic resin in order to improve pigment dispersion. Sulfonated castor oil and naphthenic soaps have also been used for this purpose, but to a lesser extent. Past efforts to modify resins in order to enhance pigment dispersion have been unsuccessful. The efforts to modify resins taken to date have resulted in resins that exhibit improved pigment dispersion, but also exhibit reduced hydrophobicity. Accordingly, there is a need for an additive for lithographic printing inks that improves pigment dispersion without adversely affecting the hydrophobicity of the resin carrier.